According to recent reports, Twitter seems to be stealing a large number of users away from Facebook for any number of reasons. Oddly enough the largest demographic is teenagers which has traditionally been the most likely to avoid Twitter but for whatever reason they're now beginning to switch over.
Many media outlets claim that privacy and security is what convinced younger users to make the switch but from here it looks like a totally different reason. Many young users are most likely transitioning simply because so many relatives (parents, grandparents, etc) have begun using Facebook extensively. As this happened many younger users began to feel like they were losing control of their secrets on Facebook. Obviously we can't say for sure this is why so many of them have switched but it definitely makes sense. Either way it is good news for Twitter as they're growing rapidly still.